50%, because, as sideshow said, the only window that has any bearing on the outcome of the game is the window which already contains the bonus. Although it occurs to me that even if the bonus window was randomly assigned, it would still work out to a 50% chance of winning.

25% is what the chance of winning would be if I presented you with four windows and said, "you have one opportunity to tell me which window has the bonus." But that's a different game altogether.

(If I wanted to be a jerk, I could point out that the 50%-by-random-guessing figure is assuming that "higher" and "lower" have been correct an equal number of times for the entire history of the game. I don't know whether that's the case.)